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Mem.S.A.It.V ol.75,282c SAIt 2004Memorie
della SimbolX https://www.doczj.com/doc/b914311007.html,astri 1,R.Gilli 1,F.Fiore 2,C.Vignali 3,1,R.Della Ceca 4,and G.Malaguti 51INAF –Osservatorio Astronomico di Bologna,Via Ranzani 1,I-40127Bologna,Italy e-mail:https://www.doczj.com/doc/b914311007.html,astri@oabo.inaf.it 2INAF –Osservatorio Astronomico di Roma,Via Frascati 33,I-00040Monteporzio Catone (RM),Italy 3Dipartimento di Astronomia,Universit`a di Bologna,Via Ranzani 1,I-40127Bologna,Italy 4INAF –Osservatorio Astronomico di Brera,Via Brera 28,I-20121Milano,Italy 5INAF–IASF,via Gobetti 101,I-40129Bologna,Italy Abstract.We will brie?y discuss the importance of sensitive X–ray observations above 10keV for a better understanding of the physical mechanisms associated to the Supermassive Black Hole primary emission and to the cosmological evolution of the most obscured Active Galactic Nuclei.Key words.Galaxies:active –X-rays –Cosmology:observations 1.Introduction The fraction of the hard X-ray background (XRB)resolved into discrete sources by deep Chandra and XMM-Newton observations smoothly decreases from practically 100%be-low 2–3keV to about 50%in the 6–10keV energy range (Worsley et al.2005),becoming negligible at energies above 10keV ,where the bulk of the energy density is produced.The shape and intensity of the unresolved XRB calculated by Worsley et al.(2005)is well described by a “peaky”spectrum which is similar to that computed by folding the aver-
age spectrum of heavily obscured (N H >1023
cm ?2)and Compton Thick (N H >1024cm ?2;
hereinafter CT)AGN over the redshift range
z ～0.5–1.5(e.g.,Comastri 2004a).
Comastri et al.:SimbolX&XRB283
CT AGN is able to penetrate the obscuring gas and is visible above～10keV,while Compton down-scattering mimics absorption over the entire energy range for heavily CT AGN.As far as the XRB is concerned,the most relevant contribution is expected to come from mildly CT AGN which are expected to be as bright as unobscured AGN above10–15keV.Though the?ux of the primary emission is strongly de-pressed,also heavily CT AGN may provide some contribution to the XRB which depend on their scattering/re?ection e?ciency,most likely correlated with the geometry of the ob-scuring gas.
At present,the best estimates of the ba-sic properties of mildly CT AGN rely on the observations obtained with the PDS in-strument onboard BeppoSAX(Risaliti et al. 1999;Guainazzi et al.2005).CT absorption, at least among nearby AGN,appears to be rather common.The relative fraction depends on the adopted selection criterium and can be as high as50–60%for optically selected Seyfert2galaxies.More recently,BAT/Swift (Ajello et al.2007)and IBIS/INTEGRAL(i.e., Bird et al.2007)have surveyed the hard X–ray sky.Although these surveys have provided relatively large numbers of hard X–ray sources (e.g.,Krivonos et al.2007),they are limited to bright?uxes(S10?100keV>10?11erg cm?2s?1), thus sampling only the very local Universe. As a consequence,the resolved fraction of the hard(>10keV)XRB is of the order of a few percent(Sazonov et al.2007).
Assuming the absorption distribution de-termined in the local Universe and folding it with cosmological evolution in AGN synthe-sis models,it is possible to predict the rel-ative number of CT AGN in a purely hard X–ray selected sample.Though the error bars su?er from small number statistics,there is a fairly good agreement with the model predic-tions(Figure1),implying that CT absorption should be common also at high redshift and luminosities.However,only a few CT AGN are known beyond the local Universe(e.g. Comastri2004b;Della Ceca et al.2007).Even the deepest X–ray surveys in the2–10keV band(e.g.,Tozzi et al.2006;Georgantopoulos et al.2007)uncovered only a dozen of candi-date heavily obscured CT AGN and await for better spectroscopic X–ray data for a con?rma-tion.
It has been recently suggested that selec-tion via mid–IR and optical colors is promising to pick up high-z heavily obscured AGN(Fiore et al.2007a;Daddi et al.2007).Stacking the Chandra counts of sources selected on the ba-sis of mid infrared(24μm)excess emission wrt the near–infrared optical?ux,a strong signal is revealed in the hard band(up to4–6keV), which implies,at the average source redshift (z～2),N H～1024cm?2.Despite the similarities in the source selection and stacking techniques, the estimated space density of the candidate high-z CT AGN di?ers by up a factor3–5.The di?erence can be ascribed at least in part to the slightly di?erent luminosity ranges sampled. According to Fiore et al.(2007a)the“miss-ing”population of luminous(L X>1044erg s?1)candidate CT AGN at z～2would have the right size to explain the unresolved XRB, while following Daddi et al.(2007)it may be signi?cantly more numerous than predicted by synthesis models especially at low(L X<1043 erg s?1)luminosities.The above estimates are a?ected by large uncertainties,however they strongly point towards the existence of a siz-able population of accreting heavily obscured SMBHs at high redshift.The direct detection of the CT AGN population over the redshift range most densely populated by less obscured sources contributing to the XRB(z～0.7–1)bears important implications for our under-standing of SMBH evolution.Moreover,a re-liable determination of their luminosity func-tion is a key parameter to reconcile the relic SMBH mass function with the local one esti-mated through the local M BH?σ/M BH?M bulge relationships and galaxies luminosity function (Marconi et al.2004;Merloni et al.2004). 2.Current hard X–ray observations The expected fraction of CT AGN in the15–200keV band as a function of?ux is reported in Figure1.The advantage of the hard X–ray selection wrt the2–10keV band is evident. The INTEGRAL and Swift all sky hard X–ray surveys have provided the?rst?ux limited
284Comastri et al.:SimbolX&
XRB
Fig.1.The predicted fraction of CT AGN(adapted
from Gilli et al.2007)as a function of X-ray?ux in
two di?erent energy ranges,as labeled.The points
with error bars correspond to the estimates in the
Chandra Deep Field South(CDFS;Tozzi et al.
2006)at faint?uxes,and from Swift/BAT survey
(Markwardt et al.2005).The upper point is com-
puted assuming that the unidenti?ed sources are all
CT AGN.
samples which can be compared with model
predictions.There is a fairly good agreement,
but the statistics is still dominated by?uctu-
ations associated to small numbers.Moreover,
the CT nature of Swift and INTEGRAL sources
is estimated combining low signal to noise
ratio hard X–ray(>10keV)spectra with
observations at lower energies,mainly with
Chandra and XMM–Newton.Follow–up ob-
servations of candidate CT AGN from the sur-
veys described above were performed with
Suzaku.The hard X–ray detector and the XIS
CCD camera onboard the Japanese satellite
are sensitive over a broad energy range(～
0.5–60keV)and return good quality spec-
tra for relatively bright hard X–ray sources.
Interestingly enough,Suzaku follow–up obser-
vations of hard X–ray selected INTEGRAL and
Swift sources(Ueda et al.2007;Comastri et
al.2007),only barely detected below10keV,
suggest that a population of extremely hard
sources,appearing only above5–6keV,may
have escaped detection by surveys at lower
energies.Their high energy spectrum(>5–
10keV)is dominated by a strong re?ection
component from optically thick material,while
there is no evidence for the presence of a soft
X–ray component,presumably due to scat-
tering of the nuclear radiation,and common
among Seyfert2galaxies.A possible expla-
nation is that the geometrical distribution of
the obscuring/re?ecting gas is di?erent from
known Type2AGN.
By requiring that their integrated emissiv-
ity does not exceed the XRB level and asso-
ciated uncertainties at30keV(e.g.Churazov
et al.2007;Frontera et al.2007),and assum-
ing the same evolution of X–ray selected AGN
(e.g.Hasinger et al.2005;La Franca et al.
2005)it is,in principle,possible to constrain
their number density.The hypothetic popula-
tion of re?ection dominated AGN could be up
to a factor4more numerous than CT AGN
(Figure2).Such an estimate is highly uncer-
tain and strongly depends upon the assump-
tion of a re?ection dominated spectrum.The
bottom line of such an exercise is that there
might be room for a previously unknown pop-
ulation of obscured AGN emerging only above
5–10keV.
3.Future imaging X–ray surveys
Imaging observations above10keV will o?er
a unique opportunity to address at least some
of the issues mentioned above.The expected
SimbolX capabilities are such to make possi-
ble the detection of a statistically signi?cant
sample of heavily obscured and CT AGN up to
z～1.The expected quality of the spectral data
over the range of?uxes accessible to SimbolX
is extensively discussed in a companion paper
(Della Ceca et al.2007).
The expected number counts in the10–40
keV band(Figure3)are obtained extrapolat-
ing the spectral energy and absorption distribu-
tions of the Gilli et al(2007)synthesis model.
The hard X–ray source surface density keeps a
steep(close to Euclidean)slope down to the
expected sensitivity limits of SimbolX obser-
vations(～10?14erg cm?2s?1)in the10–40
keV band.More speci?cally,a few hundreds
CT AGN per square degree are predicted at
Comastri et al.:SimbolX &XRB
285
Fig.2.The contribution of re?ection dominated
AGN in the Gilli et al.(2007)model (lower black
line)can be increased up to a factor 4(upper black
line)and still be consistent with present uncertain-
ties on the XRB level.
the limits of a deep survey.By scaling the es-
timated space densities for the SimbolX ?eld
of view,our estimates translate in a few up to
about ten CT AGN in a deep (1Msec)point-
ing.Taking into account the o ?-axis sensitiv-
ity and the fact that the faintest sources will be
detected with a number of counts insu ?cient
for X–ray spectral analysis,the search for the
most obscured AGN cannot be pursued only
with deep pointings.A comprehensive under-
standing of the physics and evolution of dis-
tant and obscured AGN,as well as of di ?er-
ent classes of cosmic sources,requires a close
synergy between deep and wide area surveys.
Indeed,the evolution of X–ray selected AGN
was determined by a large number of surveys
which have extensively sampled the solid angle
vs.depth discovery space (see Fig.1of Brandt
&Hasinger 2005).A possible strategy is dis-
cussed in the Fiore et al (2007b)companion
paper.
The predicted redshift distribution of CT
AGN at three di ?erent limiting ?uxes is re-
ported in Figure 4.Not surprisingly,the peak
moves to higher redshifts as the sensitivity in-
creases.A pronounced high redshift (z ～
0.5–Fig.3.The model predicted logN–logS in the 10–40keV band.The number counts for di ?erent ab-sorption column densities are reported with the la-beled colour code.The two vertical lines corre-spond to the expected limiting ?uxes for the present SimbolX con?guration:～6×10?15cgs on-axis and ～10?14cgs for an o ?-axis angle of 5arcmin.2)tail starts to develop at ?uxes around 10?13erg cm ?2s ?1and lower.As far as the search for obscured accretion at high redshift is con-cerned,the best trade–o ?would be to maxi-mize the area covered close these ?uxes.4.Conclusions Deep SimbolX surveys will be able to resolve about half of the XRB in the 10–40keV energy range (Figure 5).This achievement will repre-sents a major step forward.In fact,at present,less than a few percent of the XRB in that band is resolved.A comparison with an extrap-olation of an AGN synthesis model (Comastri 2004a)tuned to reproduce the resolved back-ground below 10keV is reported in Fig 5.Although the level of the XRB predicted to be resolved by SimboX is close to the model ex-trapolation,it will be possible,with the present con?guration,to probe the entire range of ab-sorption of the XRB sources.Several “new”CT AGN (see Fiore et al.2007b),which are undetected even in the deepest XMM–Newton and Chandra surveys (Fig.1),will be re-
286Comastri et al.:SimbolX &
XRB
Fig.4.The redshift distribution of CT AGN for
three di ?erent limiting ?uxes (10?12?10?13?10?14
erg cm ?2s ?1)from bottom to top.
vealed.Moreover,given that the fraction of
CT AGN is expected to steeply increase just below ～10?14erg cm ?2s ?1,it would be
highly rewarding to push the limiting ?ux to
somewhat lower ?uxes.It is worth remarking
that the resolution of a signi?cant fraction of
the XRB has always brought signi?cant ad-
vances in our understanding of AGN evolution.
ROSAT deep surveys showed that luminous un-
obscured quasars at high redshift (z ～1.5–2)
were responsible for most of the soft (around
1keV)XRB (Lehmann et al.2001).Later on,
thanks to Chandra and XMM–Newton deep
surveys,it was demonstrated that the bulk of
the XRB at least up to 5–6keV is originating
in relatively low luminosity sources,most of
them obscured,at z ～1(Brandt &Hasinger
2005).While we expect to uncover the so far
elusive population of CT AGN up to relatively
high redshift,it may well be possible that the
content of the X–ray sky above ～10keV is
di ?erent from what predicted.We look forward
to new unexpected ?ndings which could be ob-
tained by pushing imaging observations in the
so far unexplored hard X–ray band.
Acknowledgements.We acknowledge ?nancial
contribution from contracts ASI–INAF I /023/05/0,ASI–INAF I /088/06/0and PRIN–MUR grant 2006–02–5203.References Ajello,M.et al.2007,ApJ,in press (arXiv:0709.4333)Bird A.J.,et al.2007A&AS170,175Brandt,W.N.,Hasinger,G.2005,ARA&A,43,827Churazov,E.et al.2007,A&A,467,529Comastri,A.2004a,in Multiwavelength AGN Surveys;proceedings of the Guillermo Haro Conference held December 8-12,2003,in Cozumel,Mexico.R.Mujica and R.Maiolino eds.World Scienti?c Publishing Company,Singapore,p.323Comastri,A.2004b,in “Supermassive Black Holes in the Distant Universe”,Astroph.and Space Science Library,308,p.245Comastri A.,Gilli R.,Vignali C.,et al.2007in “The Extreme Universe in the Suzaku Era”,in press (arXiv:0704.1253)Daddi, E.et al.2007,ApJ,in press (arXiv:0705.2832)Della Ceca,R.et al.2007,these proceedings (arXiv:0709.3060)Fiore, F.et al.2007a,ApJ,in press (arXiv:0705.2864)Fiore,F.et al.2007b,these proceedings Frontera F.et al.,2007ApJ,666,86Georgantopoulos,I.Georgakakis,A.Akylas,A.2007A&A,466,823Gilli,R.,Comastri A.,&Hasinger G.2007A&A,463,73Gruber,D.E.,1999ApJ,520,124Guainazzi,M.Matt,G.Perola,G.C.2005A&A,444,119Hasinger,G.,Miyaji,T.,Schmidt,M.S.2005A&A,441,417Krivonos,R.et al.2007A&A,submitted (arXiv:astro–ph /0701836)La Franca,F.,Fiore,F.,Comastri,A.,et al.2005ApJ,635,864Lehmann,I.,et al.2001A&A,371,833Marconi,A.Risaliti,G Gilli,et al.2004MNRAS,351,169Markwardt,C.B.et al.2005ApJ,633,L77Merloni A.,Rudnick G.,Di Matteo T.,2004MNRAS,354,L37
Comastri et al.:SimbolX&XRB287
Fig.5.The fraction of the10–40keV XRB which will be resolved by a deep1Ms SimbolX pointing(point with error bars).The XRB measurements below10keV are from Chandra and XMM–Newton as well as the resolved fraction in deep?elds(gold points and red squares).while the points extending up to～15keV are from RXTE(Revnivtsev et al.2003).At high energy,the HEAO1–A4spectrum(Gruber et al.1999;green points)is reported along with the recent BeppoSAX PDS observations(Frontera et al.2007;red points). The red solid line represents an AGN synthesis model(see Comastri2004a)which reproduces the observed level of the resolved XRB below10keV.
Risaliti G.,Maiolino R.,Salvati M.1999ApJ,
522,157
Revnivtsev M.,et al.2003ApJ,522,157
Sazonov S.,et al.2007A&A,submitted
(arXiv:0708.3215)
Tozzi,P.et al.2006A&A,451,457
Ueda,Y.et al.2007ApJ,664,L79
Worsley,M.A.et al.2004MNRAS,352,L28